This invention relates to the field of winding mechanisms and more particularly to constant torque winches.
There are many applications for winding mechanisms which require the application of torque to a cable coupled to a time-varying load, where the torque amplitude is required to be relatively constant and independent of the winding speed of the mechanism. Such applications include, for example, pipe tensioners and constant tension winches. Another such application relates to the transfer of heavy loads at sea, for example, between two vessels, or between a vessel and a stationary platform, such as an off-shore oil drilling platform. Such transfers are extremely difficult in open sea environments, particularly during periods of relatively rough seas. Under these conditions, substantially constant torque must be maintained on the winch drum during load transfer operations in order to avoid breaking of the cables during sudden and large shifts of position between the vessels.
In the prior art, there have been several approaches to provide relatively constant torque winches. In one such approach, a single phase (a.c.) induction motor utilizes a non-excited squirrel cage rotor driven at constant speed and a stator driven with an alternating current of variable frequency in order to vary its speed and direction of rotation. While providing constant torque winding operation, this configuration is limited to relatively low power applications and requires relatively complex and correspondingly expensive components, both for the a.c. motor configuration and for the control circuitry.
In an alternative prior art approach, an eddy current electro brake configuration is used to drive a winding reel. The electro brake includes a rotor attached to the reel and has a coil driven by constant current, and further includes a stator driven at constant speed in which eddy currents are developed to absorb energy and brake the rotor. This configuration too has practical limitations due to power losses from heat buildup in the electro brake, and in addition, requires relatively complex and expensive motor and control circuit components.
It is an object of the present invention to provide a winding mechanism characterized by constant output torque.
Another object of the present invention is to provide a winding mechanism characterized by a preselected output torque determined by armature and field currents of a d.c. motor.